(1) Field of the Invention
The present invention relates to a multi-cylinder internal combustion engine which is adapted to keep some of the cylinders thereof inoperative while the rest of them are operating when the magnitude of the load on the engine decreases.
(2) Description of the Prior Art
In a motor vehicle such as an automobile and the like, all cylinders of a multi-cylinder engine must be operated simultaneously to provide sufficient output power for driving under a heavy load condition. It, however, is often experienced that if all the cylinders are continued to operate simultaneously even after the engine load becomes small, that is, during a partial load condition, each cylinder tends to reduce its charging efficiency, resulting in increased pumping loss and decreased combustion efficiency and fuel economy.
To overcome such shortcomings, several new solutions are already proposed in the prior art, as described, for instance, in the Japanese Patent Application No. 288770/1975 wherein some cylinders are made inoperative under a partial load condition on the engine. This enables a concentrated fuel-air mixture to enter the rest of cylinders, thereby providing a consequential improvement in the fuel/milage ratio as a result of increased combustion efficiency and reduced pumping loss. Some cylinders, in such prior art, can be made inoperative by retaining their intake valve and exhaust valve in completely closed positions, thus interrupting further inflow of the fuel-air mixture to the cylinder.
However, such a prior art solution is not satisfactory because the air left in the inoperative cylinders is repeatedly compressed and expanded every revolution of the engine. Thus, the driving torque is subject to large fluctuations when the inoperative pistons perform intake and exhaust strokes, although the positive and negative driving torques would be small during the same strokes if the cylinders became operative. This, along with the decreased number of operating cylinders, inevitably causes the problem that smooth engine rotation is significantly disturbed due an increase in a primary vibrational component as a result of abnormally large fluctuations of the driving torque generated by the inoperative cylinders.